1. Field of the Invention
The present invention relates to the field of supplying direct current (DC) power; and specifically, to filtering noise between the power supply and the load driven by the power supply.
2. Discussion of the Related Art
FIG. 1 illustrates a conventional filter for isolating an electrically noisy load from a low noise main power supply source. The conventional circuit illustrated in FIG. 1 is used primarily to filter an electrically noisy load 101 such as a switching DC to DC converter, brushless DC motor, switching logic circuit, or another noisy direct current powered circuit from a low noise main power source, VIN. Inductor L1 acts as a passive low pass filter for isolating a low noise main power source, VIN, from noise generated by the noisy load.
The inductance value of inductor L1 is chosen to optimize the noise filtering requirements. In designing the TLE conventional circuit such as illustrated in FIG. 1, care must be exercised in the choice and physical placement of inductor L1 as it may emit magnetic noise interference, and it may be physically large. Magnetic noise interference may undesirably affect the performance of the load device, the power supply, and other unrelated electronic/magnetic devices in the physically nearby area of the system illustrated in FIG. 1.
An object of the present invention is to create an active power filter which behaves like a filter circuit having a large inductor, but without producing magnetic interference or physically large components as would be produced by a conventional large inductor. The power filter according to the present invention is suitable for use with switching DC to DC converters, brushless DC fans, switching logic circuits, and other noisy loads.
According to the present invention, an active power filter includes a feedback resistor and a shunt capacitor, an operational amplifier equivalent subcircuit, and a voltage drop source. The shunt capacitor connects the positive terminals of the low noise power supply and the noisy load to the positive terminal of the operational amplifier equivalent subcircuit. The feed back resistor connects the negative terminal of the noisy load and the output of the operational amplifier equivalent subcircuit to the negative terminal of the shunt capacitor. The voltage drop source connects the negative terminal of the low noise power supply to the negative terminal of the operational amplifier equivalent subcircuit.
According to the present invention, the impedance of the active power filter at a minimum noise frequency is carefully designed so as to be large in comparison to an impedance of the noisy load, for example, the impedance of the active power filter is 1000 times the impedance of the noisy load at the minimum noise frequency. The active power filter according to the present invention is equally capable of protecting a low noise load device to a noisy direct current power supply.
According to another aspect of the present invention, the operational equivalent subcircuit includes an operational amplifier, first and second resistors, and a transistor. The first resistor connects the positive terminal of the voltage drop source to the negative input terminal of the operational amplifier. The second resistor connects the output of the operational amplifier to the gate of the transistor. The drain of the transistor is coupled to the negative terminal of the noisy load. The source of the transistor is coupled to the negative terminal of the voltage drop source and to the negative terminal of the low noise direct current power supply. The positive input terminal of the operational amplifier forms the positive input terminal of the operational amplifier equivalent subcircuit.
In an exemplary embodiment of the active power filter according to the present invention, the operational amplifier equivalent subcircuit further includes, first and second capacitors. The first capacitor connects the output of the operational amplifier to the negative input terminal of the operational amplifier. The second capacitor connects the output of the operational amplifier to the gate of the transistor. In addition, the operational amplifier equivalent subcircuit may contain a third capacitor and a third resistor which are connected in series between the gate of the transistor and the negative terminal of the low noise direct current power supply. In an exemplary embodiment, the transistor comprises an N-channel enhancement mode MOS field effect transistor.
These and other features, aspects, and advantages of the present invention are more fully described in the Detailed Description of the Invention with reference to the Figures.